The present invention relates to a thin film capacitor and a method of forming the same, and more particularly to a thin film capacitor with an improved top electrode suitable for advanced semiconductor devices and advanced integrated circuits and a method of forming the same at high throughput and high yield.
As a degree of integration of semiconductor memory devices such as dynamic random access memories has been on the increase, various kinds of high dielectric oxide such as (Ba, Sr)TiO.sub.3 have been on active investigations for a dielectric film of a thin film capacitor, instead of silicon dioxide or silicon nitride.
When the high dielectric oxide is used for the dielectric film of the thin film capacitor, a polysilicon electrode is not useable due to a problem with possible oxidation of an interface of the polysilicon electrode with the high dielectric oxide film, for which reason metals such as Pt, Ru and conductive metal oxide as well as conductive nitrides such as TiN have also been on the investigation for electrodes of the thin film capacitor.
The thin film capacitors having the high dielectric oxide layer sandwiched between top and bottom electrodes have been investigated and reported mostly about improvements in capacitance characteristic and current leakage characteristics both of which are important factors for the thin film capacitors.
A first prior art about the top electrode of the thin film capacitor was reported in Japan Applied Physics Association 1997, No. 2, p. 398. The thin film capacitor has a Ru/(Ba, Sr)TiO.sub.3 /Ru structure, wherein a (Ba, Sr)TiO.sub.3 dielectric layer is deposited by a metal organic chemical vapor deposition method whilst Ru top and bottom electrodes are deposited by a DC sputtering method. Good capacitance characteristics and current leakage characteristics can t)c obtained such that a silicon conversion thickness is 0.34 nanometers and a density of a leak current is in the order of 1.times.10.sup.-8 A/cm.sup.2.
A second prior art of the thin film capacitor was reported in Japanese Journal of Applied Physics, Vol. 36, No. 9B, pp. 5860-5865. The thin film capacitor has a Pt/(Ba, Sr)TiO.sub.3 /Pt structure, wherein a (Ba, Sr)TiO.sub.3 dielectric layer is deposited by a radio frequency magnetron sputtering method whilst Pt top and bottom electrodes are deposited by a DC sputtering method. There was investigated variations in current leakage of the thin film capacitor over deposition conditions for a second DC sputtering process for the Pt top electrode It was confirmed that under application of a voltage of -1V, the leakage of current at a DC power of 0.2 kW is smaller by two or three digits than at DC powers of 0.5 kW and 1.0 kW. The reason why the leakage of current is reduced is that the roughness of an interface of the top electrode with the high dielectric oxide film reduces a Schottky barrier height of the interface between the top electrode and the high dielectric oxide film.
However, other prior arts than the above are directed to the high dielectric oxide layers and the bottom electrode structures whilst the prior art directed to the top electrode structure is rare case. The bottom electrode and the high dielectric oxide layer are likely to be strongly influenced by later processes, for which reason most of the development and investigation was directed to the bottom electrode and the high dielectric oxide layer. On the other hand, the top electrode is in major cases grounded, for which reason the top electrode has received weak attention or concern.
The present inventor has investigated possible various factors of the top electrode which might provide influences to the thin film capacitor and could confirm the fact that an interface state between the high dielectric oxide layer and the top electrode provides large influences to the current leakage characteristic and the adhesion between them for the thin film capacitor.
For example, the above first prior art thin film capacitor of the Ru/(Ba, Sr)TiO.sub.3 /Ru structure shows the good leakage characteristic of 1.times.10.sup.-8 A/cm.sup.2. Notwithstanding, the first prior art thin film capacitor of the Ru/(Ba, Sr)TiO.sub.3 /Ru structure was placed in oxygen gas or nitrogen gas at 500.degree. C. for 30 minutes in order to have confirmed a temperature hysteresis, whereby the leakage characteristic is deteriorated. The causes of the deterioration in the current leakage characteristic has been investigated with TEM observation and local EDX analysis and could confirm the fact that the deterioration in the current leakage characteristic is caused by both oxidation of Ru on an interface between the Ru top electrode layer and the (Ba, Sr)TiO.sub.3 high dielectric oxide layer and a diffusion of Ru from the Ru top electrode layer into the (Ba, Sr)TiO.sub.3 high dielectric oxide layer.
In general, the top electrode as formed receives a beat treatment such as an anneal at a temperature of not less than 350.degree. C. for formation of interconnections extending over the thin film capacitor or a passivation film overlying the thin film capacitor, for which reason if a highly oxidizable metal is used for the top electrode, then a thermal oxidation of the metal may appear whereby the current leakage characteristic is deteriorated.
It was also confirmed by the present inventor that if the top electrode was deposited by a sputtering method under a condition of a high voltage application to a sputter target, then the oxidation of Ru and the diffusion of Ru from the top electrode into the (Ba, Sr)TiO.sub.3 high dielectric oxide layer could be observed even though no heat treatment was carried out.
Meanwhile, the above second prior art shows the fact that the current leakage characteristic could be improved by two or three digits by drop of a power of the DC sputtering process for reduction in deposition rate. Table 1 on page 5860 of Japanese Journal of Applied Physics shows that in order to obtain the improvement by two Or three digits of the current leakage characteristic, it is required to remarkably reduce the deposition rate of the top electrode down to about one quarter. Such remarkable reduction in deposition rate causes a undesirable reduction in throughput, whereby productivity of the thin film capacitor is thus dropped.
An adhesiveness of the second prior art thin film capacitor of the Pt/(Ba, Sr)TiO.sub.3 /Pt structure was evaluated and it was confirmed that the adhesiveness of the film is lowered by drop of the power applied to the target during the DC sputtering process. Such reduction in the adhesiveness of the film increases a probability of peeling the film, whereby reliability of the semiconductor device and the yield thereof are thus reduced.
It is preferable to use Pt for the top electrode in light of prevention of oxidation at the top electrode interface with the high dielectric oxide layer and of the diffusion of Pt into the high dielectric oxide layer. The use of Pt for the top electrode, however, raises a problem with irradiation of alpha-ray from Pt. Namely, if the Pt top electrode layer is thick, then a sufficiently large amount of alpha-ray for causing a soft error is irradiated from the thick Pt top electrode layer. This means that so long as Pt is used for the top electrode, it is required to form a thin Pt top electrode in order to prevent the soft error.
In addition, Japanese laid-open patent publication No. 7-221197 addresses a Ru bottom electrode for the thin film capacitor but is silent on the structure of the top electrode.
Further, Japanese laid-open patent publication No. 8-17806 addresses a method of forming a thin film capacitor having the Pt/(Ba, Sr)TiO.sub.3 /Pt structure but is silent on the structure of the top electrode and on any influence to the capacitance characteristic by use of Pt for the top electrode.
In the above circumstances, it had been required to develop a novel thin film capacitor with an improved top electrode structure which makes the thin film capacitor free form any problems and disadvantages as described above and a novel method of forming the same.